Abstract

In a semiconductor optical amplifier (SOA) with copropagating optical pump pulses, the application of a nonlinear phase shift to optical signals provides the driving force for all-optical interferometric switching. We study, both analytically and experimentally, the dependencies of the nonlinear phase shift on the driving frequency (42–168 GHz) and on the SOA parameters. We have found that the nonlinear phase shift (ΔΦNL) decreases with the driving frequency but that this decrease is only linear, i.e., ΔΦNL∝f-1. We have also found that the nonlinear phase shift in the SOA linearly increases with the injection current (Iop), i.e., ΔΦNL∝Iop, even in this ultrahigh-frequency range.

Schematic view of the nonlinear phenomena inside a SOA: (a) the change in the carrier density that is induced by the optical control pulses, which causes (b) a nonlinear change in refractive index and (c) a change in the gain. The dashed curves in (a) indicate the waveforms of the input pulses.

Mechanism in the SMZ-type all-optical switches: (a) Each of the two split signal components inside the interferometer obtains a nonlinear phase shift. (b) The phase difference between the two components and (c) the switching window.

Comparison of the DISC output (solid curve) and the SOA output (dashed curve) as observed with a streak camera, showing that the SMZ-type switch can form a switching window whose width is much shorter than the carrier lifetime.

Measured (solid curves) and calculated (dashed curves) autocorrelator traces (a), (c) and spectra (b), (d) of 42-GHz 7.0-ps wavelength-converted pulses at the output of the DISC: (a), (b) with the phase bias (ΔΦb) set to π and (c), (d) with ΔΦb optimized to 1.10 π. The dotted curves in (b) and (d) are the calculated spectrum of a transform-limited 4-ps pulse, which is given as a reference.

Correlation of the calculated intensity ratios between components of the XPM spectrum with the calculated nonlinear phase shift induced by pulses at 42 GHz (triangles) and 168 GHz (circles). The dashed line in (a) is a line of fit to the set of data points. The dashed curve in (b) was calculated with Eq. (22) of the text.

Typical eye diagram after 168-GHz random switching (wavelength conversion). The eye diagram was measured after demultiplexing of the 168-Gbit/s wavelength-converted output to 10.5-Gbit/s signals because of the limited bandwidth of our sampling scope.

Observed pulse waveforms and spectra under the wavelength-conversion conditions: For the input pulses, (a) a streak-camera image and (b) an autocorrelation trace, and for the pulses at the DISC output, (c) a streak-camera image, (d) an autocorrelation trace, and (e) an XPM spectrum. The nonlinear phase shift involved in this operation was measured as 0.2–0.3π on the basis of the XPM spectrum observed at the SOA output.